Third Harmonic Generation Phenomenon
Third harmonic generation is a well known nonlinear optical phenomenon (see for example R. W. Boyd, Nonlinear Optics, Academic Press, San Diego, 1992). Briefly, nonlinearities in the response of a material to an illumination laser field give rise to new frequency components of the optical field that correspond to the third harmonic of the frequency of the illumination field. It is known that third harmonic signals can be generated by focusing a laser beam close to material interfaces, and to thin layers embedded within media. This phenomenon of surface third harmonic generation has been demonstrated experimentally for a variety of materials and interfaces (see for example T. Y. F. Tsang, “Optical third-harmonic generation at interfaces”, Physical Review A, 52 (5), 4116, November (1995)).
Surface third harmonic generation has found applications in nonlinear microscopy (See for example Y. Barad, H. Eisenberg, M. Horowitz, and Y. Silberberg, “Nonlinear scanning laser microscopy by third harmonic generation”, Applied Physics Letters, 70 (8), 24, 922 (1997), D. Yelin, D. Oron, E. Korkotian, M. Segal, and Y. Silberberg, “Third-harmonic microscopy with a titanium-sapphire laser”, Applied Physics B, 74, S97 (2002)).
U.S. Pat. No. 5,828,459 of Silberberg describes a scanning laser microscope that uses third harmonic generation for acquiring high resolution images.
Briefly, when radiation beam having a fundamental frequency ω is focused onto an interface defined by two materials that are characterized by different χ(3) (third order nonlinear susceptibility) third harmonic signals (signals having a frequency of 3ω) are generated.
Surface third harmonic generation has also found applications in ultrashort pulse characterization (see for example T. Tsang, M. A. Krumbügel, K. W. DeLong, D. N. Fittinghoff, and R. Trebino, “Frequency-resolved optical-gating measurements of ultrashort pulses using surface third-harmonic generation”, Optics Letters, 21 (17), 1381 (1996), and D. Meshulach, Y. Barad, and Y. Silberberg, “Measurement of ultrashort optical pulses by third-harmonic generation”, Journal of Optical Society of America B, 14 (8), 2122 (1997)).
Integrated Circuit Lithography
Integrated circuits are very complex devices that include multiple layers. Some layers may include conductive material and isolating material while other layers may include semi-conductive materials. These various materials are arranged in patterns, usually in accordance with the expected functionality of the integrated circuit. The patterns also reflect the manufacturing process of the integrated circuits.
Integrated circuits are manufactured by complex multi-staged manufacturing processes. During these multi-staged processes, radiation resistive (usually termed photoresist) material is deposited on a substrate/layer, exposed by a photolithographic process, and developed to produce a pattern that defines some areas to be later etched. After the pattern is etched, various materials, such as copper are deposited. The deposition process is usually followed by a polishing process. Materials can be polished by chemical processes and/or a combination of chemical as well as mechanical processes.
The size of the features of integrated circuits continues to shrink and there is a growing need to provide methods and systems for improving the resolution and reducing the size of printed features of lithography systems and lithography methods.